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UPS Runtime and Sizing Calculator

Runtime for a UPS at a given load, and the VA rating and battery capacity to specify for a target runtime.

W

Modern computer supplies are around 0.95–0.99. Older kit can be 0.6–0.7.

%

Headroom above the load. Running a UPS near its limit shortens battery life and leaves no room for growth.

V

12 V for small units, 24 or 48 V for larger ones.

Ah

From the battery label. 7–9 Ah is typical in a desktop UPS.

%
%

UPS units almost always use sealed lead-acid, where 50% is the working figure.

Runtime at this load
— min

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UPS rating needed — VA
Usable energy in the battery — Wh
Drawn from the battery — W
Reading this —

The formula

VA = watts ÷ power factor × (1 + headroom) usable Wh = battery V × Ah × usable fraction minutes = usable Wh ÷ (load ÷ inverter efficiency) × 60

UPS units are rated in VA because that is what their inverter and transfer switch are limited by, but the battery is drained in watts. The two are related by the power factor of your load, which is why a 1,000 VA unit may only support 600 W.

What to watch out for

The VA and watt figures on a UPS are not the same number, and the smaller one binds:

  • VA is the inverter limit. A 1,500 VA unit is often rated 900 W — a power factor of 0.6 assumed at the design stage.
  • Watts is what drains the battery. Runtime scales with watts, not VA.

Runtime falls away faster than linearly as load rises, because lead-acid loses effective capacity at high discharge rates. A unit rated 20 minutes at half load may manage only 6 or 7 at full load.

Batteries age. A sealed lead-acid UPS battery holds most of its capacity for two or three years and then falls off quickly — a four-year-old unit may have a third of its original runtime, with no warning until the power goes.

Frequently Asked Questions

How long will a UPS run my computer?

A typical 24 V, 9 Ah desktop unit holds about 108 Wh usable. A 400 W load draws roughly 444 W through the inverter, giving around 15 minutes — enough for a clean shutdown, not to keep working.

What size UPS do I need?

Divide your load in watts by its power factor to get VA, then add 20–25% headroom. A 400 W load at 0.9 power factor is 444 VA, so a 600 VA unit or larger.

Why is a 1,500 VA UPS only rated 900 W?

Because VA and watts differ by the power factor, and manufacturers assume a conservative one. The unit cannot exceed either figure, so the watt rating is usually the one that limits you.

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